The Archaea are one of the three major groups of living organisms, together with the bacteria and eukaryotes. They are prokaryotes like the bacteria, and were originally included among them. Their separate identity was discovered in the late 1970s by Dr. Carl Woese at the University of Illinois by genetic comparison. Originally they were termed the Archaebacteria, and the other prokaryotes the Eubacteria, but now there is a growing tendency to restrict the term bacteria to the latter and the names have adjusted accordingly. The Archaea may be treated as a single kingdom or as a domain, in which case the subgroups may be ranked as kingdoms.

Archaea differ from the true bacteria in many important respects, as well as from the eukaryotes. These differences include:

wall structures and chemistry (lack of peptidoglycan and gram staining)

Many Archaea live in extreme environments, including water whose temperature exceeds that of boiling water, like geysers, very salty, acid or alkaline water or black smokers. They are very diverse, both in morphology and physiology. Some are single-celled, while others form filaments or aggregates. They may be spherical, rod-shaped, spiral, lobed. Their size varies in diameter from 0.1 to over than 15 µm (filaments up to 200 µm).

They show a great diversity in multiplication modes, which may be by binary fission[?], budding[?] or fragmentation[?].
For a nutrional point of view, they range from being chemolithoautotrophic to organotrophic. Physiologically, they can be aerobic, facultatively anaerobic, or stricly anaerobic. Some are mesophiles[?], others hyperthermophiles (may live over 100°C). Though most of them live in high-temperature, anaerobic, hypersaline environment, some have also been found in cold places. They are mostly found in aquatic and terrestrial habitats, but a few have been found in animal digestive systems. The environmental conditions archaea prefer and their unusual biochemistry make them usually harmless to organisms belonging to the other two domains. No case of infection of a human with archaea has been reported so far.

There are two main groups of Archaea, the Crenarchaeota[?] and Euryarchaeota[?]. The Korarchaeota[?] have been described from DNA samples, but the actual organisms remain unknown, and the Nanoarchaeota are known from a single species discovered in 2002, Nanoarchaeum equitum. Some work suggests that the Euryarchaeota may be closer to the eukaryotes than the Crenarchaeota, in which case the domain Archaea would be abandoned. Microbiologists who consider the Bacteria to be paraphyletic also argue that the Archaea are not sufficiently different to be considered a separate group.